Basic Research Needs for Solar Energy Utilization

(backadmin) #1

low toxicity, and processibility. To meet these challenges, the tools of theory and experiment
must be brought together to understand and control catalytic processes.


Blueprint for Fuel-forming Catalyst Design


The essential requirements for electron-transfer driven, catalyzed production of solar fuels (such
as H 2 or a carbon based liquid fuel) are captured in Figure 43. The left-hand system features the
oxidation of water, and the right-hand side features (for purposes of illustration) reduction of
CO 2 to formic acid. Such systems share common features and illustrate a generic, systems
approach to successful catalyst design. The essential elements are as follows.


(1) An electron transfer interface. Catalyst systems for oxidation or reduction
are driven by electron transfer to or from an electron transfer interface. The
source of electrons is generic, potentially from molecular excitation-electron
transfer, a photovoltaic source, or even the excitation-electron transfer
apparatus of the natural photosynthetic apparatus.

(2) Proton-coupled electron transfer (PCET) for redox leveling and proton
addition or removal. The gain or loss of protons, which prevents the
accumulation of charge, is required for multi-electron transfer in order to
avoid high-energy proton intermediates, thereby reducing reaction barriers.

(3) Catalysis via atom, ion transfer, bond formation and breaking. The key
elements at this site are the utilization of atom (e.g., O or H) or ion (e.g., H-)
transfer reactions that carry out the complex chemical transformations
required with reaction barriers sufficiently low to ensure facile reactions on
the sub-millisecond time scale. All of these elements must be spatially
arranged to couple efficiently to ultimately generate fuel.

Blueprint for Fuel-Forming Catalyst Design
Oxidations Reductions

2H 2 O

O 2
4H+

4e–

CO 2

HCO 2 H

2e–

2H+

1 3 1 3

stepping beyond Marcus (1e–)
atom transfer
(bond-breaking/making)

(^1) multi-electron 3
transfer
(^2) proton-coupled
electron transfer
2
2
Photoinduced
Charge Separation
Assembly
Photoinduced
Charge Separation
Assembly
Blueprint for Fuel-Forming Catalyst Design
Oxidations Reductions
2H 2 O
O 2
4H+
4e–
2H 2 O
O 2
4H+
2H 2 O
O 2
4H+
4e–
CO 2
HCO 2 H
2e–
2H+
CO 2
HCO 2 H
2e–
2H+
11 33 11 33
stepping beyond Marcus (1estepping beyond Marcus (1e––))
atom transfer
(bond-breaking/making)
(^1) multi-electron 3
transfer
(^2) proton-coupled
electron transfer
atom transfer
(bond-breaking/making)
(^1) multi-electron 33
transfer
(^11) multi-electron
transfer
(^22) proton-coupled
electron transfer
222
222
Photoinduced
Charge Separation
Assembly
Photoinduced
Charge Separation
Assembly
Photoinduced
Charge Separation
Assembly
Photoinduced
Charge Separation
Assembly
Figure 43 A blueprint for catalyst design detailed for both oxidation
and reduction catalysts (Cat = catalyst)

Free download pdf